The present invention relates to a system for reducing harmonic currents on a multiphase alternating current power system due to the use of harmonic current generating loads connected to the power system. More particularly, the present invention relates to active filters used to selectively inject harmonic currents into the multiphase alternating current power system.
The use of non-linear loads such as uninterruptable power supplies (UPS), adjustable speed drives, induction heaters, arc welders and computer equipment, to name a few, has been steadily increasing. These devices are connected to the power system at a point of common coupling (PCC) and can be supplied electric power from a three-phase, four-wire distribution system with both single-phase and three-phase inputs.
Typically, a non-linear load includes a converter that is used to convert the alternating current power from the utility system to direct current power. As is commonly known, converters contribute harmonic current into the power system. These harmonic currents can interfere with communication and control signals, cause economic losses due to errors in metering and malfunctioning of utility system protection relays and stress the utility system equipment, for example, the distribution transformers, from heat generated by the harmonic currents and over-voltage conditions that can occur in resonant situations.
National and international agencies have established various standards and guidelines to specify allowable limits for harmonic currents on a system. Recently, IEEE 519-1992 was revised to recommend a limit on the total harmonic distortion (THD) in the current at the point of common coupling to be as low as 5%.
Active filters have been used in the past to control the presence of harmonic currents in the power system. Typically, the active filter includes a power electronic converter that supplies the harmonic currents present at the point of common coupling so the harmonic currents are not present on other portions of the power system. Active filters for eliminating harmonic currents in a three-phase, four-wire system have been reported in "A Four-Wire, Current-Controlled Converter Provides Harmonic Neutralization in Three-Phase, Four-Wire systems", 1993 IEEE/APEC PROCEEDINGS, pp. 841-846. Another active filter disclosed in "Analysis and Design of a New Active Power Filter to Cancel Neutral Current Harmonics in Three Phase Four Wire Electric Distribution Systems", 1993 IEEE/IAS Conference Records, pp. 939-946, only reduces the neutral-wire current harmonics.
As an alternative to using an active filter, the use of a zig-zag transformer for injecting currents in a power system is described in "Application of Zig-Zag Transformers for Reducing Harmonics in the Neutral Conductor of Low Voltage Distribution System" 1990 IEEE/IAS Conference Records. This reference discloses the use of a zig-zag transformer to reduce harmonics in the neutral path of a low voltage distribution system. Phase windings of the zig-zag transformer are connected to each of the phase conductors of the power system. The neutral of the zig-zag transformer is connected to the neutral conductor of the power system. The zig-zag transformer shares neutral current with a secondary winding of the distribution transformer in proportion to the impedance of the alternate current paths.